Operating an electrical power converter, which is used in a medium-voltage electrical system comprising the electrical converter and an electrical machine, with optimized pulse patterns may cause higher current harmonics supplied to the electrical machine. To reduce the negative effects caused by the higher harmonics, an LC filter may be installed between the output of the electrical converter and the electrical machine. While the filter attenuates the harmonic content that is fed to the electrical machine, it may introduce a resonant behavior into the electrical drive system. Usually, this resonant behavior is treated by using an additional active damping controller working independently from the main tracking controller, which selects the optimized pulse patterns to be applied to the electrical converter.
Since the electrical machine is supplied by the electrical converter with finite number of voltage levels, the performance of the electrical system during steady state may be described by Total Harmonic Distortion (THD). Its value quantifies the distortion of the stator currents from ideal sinusoidal shape. The steady state stator currents are characterized by fundamental harmonics responsible for the conversion of electrical energy to mechanical energy, and higher harmonics which are the consequence of the switching nature of the electrical converter. Higher current harmonics usually cause harmonic losses in the electrical machine. As a consequence of higher losses, electrical machines need to be oversized which leads to a higher price. Moreover, the increased losses caused by higher current harmonics may prevent the installation of power converters to DOL (Direct On-Line) electrical machines that are designed to work via a direct connection to the power grid.
One of the possibilities for reducing the steady state current distortion would be the insertion of an LC filter between the electrical converter and the electrical machine. This kind of filter may cause a very strong attenuation rate of higher stator current harmonics, and may open the possibility of obtaining much lower current distortion. However, the insertion of an LC filter may introduce a resonant behavior into the electrical system. One way to overcome the problem with the introduced resonant behavior may be the addition of a resistive element in a serial connection to a filter capacitor. This concept is known as passive damping. However, the addition of the resistive element may also cause a weaker attenuation rate of higher current harmonics provided by the filter. Moreover, the inserted resistive element R dissipates energy and therefore reduces the energy efficiency of the system.
A more suitable way to solve the problem of the resonant behavior introduced by the LC filter is by using appropriate damping control. This way, it is not necessary to insert the resistive element R and the powerful attenuation of higher current harmonics is preserved. These solutions are mostly based on an additional damping loop. The additional damping loop comprises an auxiliary controller which provides the damping correction of reference as a reaction to measurements filtered around resonant frequency.
Recently, there has been a growing interest in the application of predictive control in power electronics. The developed predictive control schemes applied to medium voltage electrical systems demonstrated a considerable improvement of performance in comparison to the previous control methods. The advantages are related both to better dynamic performance during transients, as well as better steady state behavior. For example, WO 2014 183930 A1 and WO 2014 064141 A1 relate to model predictive control of electrical converter systems.
EP 2 469 692 A1 discloses a method for controlling an electrical converter system determining a reference output and an estimated output of the electrical converter system based on measurements in the electrical converter system and determining an optimized pulse pattern by selecting from a table of precalculated optimized pulse patterns, which is chosen based on the reference output and the estimated output.